Combined riser tensioner and drill string heave compensator

ABSTRACT

The rod end of a heave compensator is hydraulically coupled to the head end of a riser tensioner used on a floating drilling vessel. The rod end of the riser tensioner is pressurized an amount sufficient to provide not only a tension on the riser, but in addition a lifting force on the drill string. The hydraulic fluid displaced between the heave compensator and riser tensioner accommodates rotation of the vessel due to wave action. The riser tensioner is isolated from the heave compensator and coupled to a pressurized hydraulic supply whenever the heave compensator is disconnected from the drill string.

United States Patent 1191 Scozzafava Primary Examiner-Marvin A. Champion Assistant Examiner-Richard E. Favreau AttorneyRichard W. Seed et a1.

[57] ABSTRACT The rod end of a heave compensator is hydraulically coupled to the head end of a riser tensioner used on a floating drilling vessel. The rod end of the riser tensioner is pressurized an amount sufficient to provide not only a tension on the riser, but in addition a lifting force on the drill string. The hydraulic fluid displaced between the heave compensator and riser tensioner accommodates rotation of the vessel due to wave action. The riser tensioner is isolated from the heave compensator and coupled to a pressurized hydraulic supply whenever the heave compensator is disconnected from the drill string.

8 Claims, 2 Drawing Figures COMBINED RISER TENSIONER AND DRILL STRING HEAVE COMPENSATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to floating drilling vessels and more particularly to apparatus and methods for maintaining substantially constant lifting forces on the drill string and the riser regardless of motion of the vessel due to wave action.

2. Description of the Prior Art The petroleum industry has been faced with increasing problems as drilling is moved to deeper waters. The high cost of operating a drilling vessel dictates that down time due to weather be minimized. Of specific concern is the wave action of floating drilling vessels with the resultant effect on operation of the drill string and support of the riser to prevent buckling. A number of prior art systems have been proposed for heave compensation, that is, the effect of wave action on loading of the drill string. Also known are proposals for preventing buckling of the riser by tensioning either by counter weight or mechanical means. All of these proposed systems, however, have utilized separate independent systems each to separately compensate for the wave action on the drill string and riser.

One of the prior independent heave compensation systems for drill string loading is shown in US. Pat. No. 3,158,206. This system employs a float coupled to a piston in an accumulator cylinder to provide a constant pressure supply for the heave compensating cylinder coupled to the drill string. Other types of independent systems are shown in US. Pat. Nos. 3,163,005 and 2,945,677.

SUMMARY OF THE INVENTION It is an object of this invention to provide a drill string heave compensator and a riser tensioner which are hydraulically coupled to compensate each other to accommodate wave action on the drilling vessel.

It is another object of this invention to provide a combined drill string compensator and riser tensioner which reduces friction line loss effects on the heave compensator and loads the drill string in a generally constant manner similar to aland based drilling rig.

It is another object of this invention to provide a heave compensator cylinder and piston hydraulically coupled to a riser tensioner such that a constant pressure is provided in the fluid in the heave compensator cylinder regardless of the position of the piston in the cylinder.

It is another object of this invention to provide a combined drill string heave compensator and riser tensioner which requires substantially less back-up equipment than is needed for independent heave compensator and riser tensioner systems.

A preferred form of the invention comprises a heave compensator device, having one or more cylinders, pistons, and piston rods which are contained in the drill string supporting mechanism.v A fluid is maintained in the rod end of the cylinder. The riser tensioner comprises one or more cylinders, pistons, and piston rods with fluid in one end of the cylinder coupled to the fluid in the heave compensator cylinder. The riser tensioner cylinder and piston rod is coupled between the riser and the drilling vessel. Gas or fluid in one end of the riser tensioner cylinder is pressurized to an amount necessary to tension the riser plus support the drill string. The drill string supporting pressure is passed through the fluid to the heave compensator cylinder. Finally, means are provided for isolating the heave compensator from the riser tensioner and coupling the drill string supporting fluid in the riser tensioning cylinder to an independent fluid supply of equal pressure to compensate for the removal of the drill string load while the riser tensioner continues to operate.

In a preferred form of the invention the heave compensator comprises only a single cylinder, piston and piston rod and the riser tensioner comprises a plurality of cylinders, pistons and piston rods with the cumulative effective cross-sectional areas of the riser tensioner cylinders being equal to the effective cross-sectional area of the heave compensator cylinder. The rod end of the heave compensator cylinder is hydraulically coupled to the head ends of the riser tensioner cylinders. The rod ends of the riser tesioner cylinders are pressurized via a fluid-gas accumulator from a large volume gas supply.

In one example of a combined heave compensator and riser tensioner a total gas pressure on the rod ends of the riser tensioner cylinders is 2,375 psi. This provides a tensioning force on the riser of approximately 700 lbs. and a pressure in the fluid in the head ends of the riser tensioner cylinders and thus the rod end of the heave compensator cylinder of approximately 1,675 psi or a lifting force on the drill string of approximately 400,000 lbs. The pressures, of course, may be varied, depending upon the length of drill string and the desired drill string lifting force as determined by the driller and likewise the desired riser tension.

The combining of the riser tensioner and drill string heave compensator is accomplished by providing isolation means to disconnect the riser tensioner cylinders from the heave compensator cylinder and to either substitute for the drill string load fluid pressure equivalent to that initially imposed by the heave compensator cylinder or to reduce the pressure in the rod ends of the riser tensioner cylinders to an amount only sufficient to maintain a tension on the riser.

Several advantages are obtained by a combined system: First, back-up equipment for a second independent system is eliminated; second, the riser couples the system to the earth, reducing spring action on the drill string to provide a constant drill bit loading; third, friction losses in the hydraulic lines are made up by piston movement in the riser tensioner cylinder thus affecting riser tension rather than drill bit loading.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a typical drilling vessel embodying the principles of the invention.

FIG. 2 is a diagrammatic illustration of a preferred hydraulic circuit embodying the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The interconnected drill string heave compensator and riser tensioner system illustrated is basically a pascables 14 to an upper crown block 16. As is well known in the art the relative movement between the traveling block and the crown block provides the lengthwise positioning of the drill string for a drilling operation. The heave compensator is provided with a piston and the piston rod 18 that is coupled to a hook mechanism 20 which rotatably supports the drill string DS. In use fluid under pressure is maintained in the rod end of the cylinder 12 to support the weight of the drill string and provide the desired drilling force on the bit in the bore hole at the bottom of the ocean. As is well understood, movement of the drilling vessel DV due to wave action is translated into movement of the cylinder 12 rather than the drill string with the hydraulic fluid below the piston maintaining a constant lifting force on the drill string.

The combined system includes a riser tensioner 24, which in the preferred embodiment comprises four cylinders 26 of cumulative effective cross-sectional area equal to the heave compensator cylinder crosssectional area and shown as an equivalent cylinder 26e in FIG. 2. In the alternative the riser tensioner cylinders could be smaller in cross-sectional area and a multiplier of the stroke provided as where one foot of movement of the vessel results in two feet of piston stroke in the riser tensioner cylinder. The important factor is that the same volume of liquid is displaced in the cumulative riser tensioner cylinders as is displaced in the heave compensator cylinder. The tensioners are provided with pistons and piston rods 28 which are coupled to a drilling riser DR in a conventional manner. As is well understood the drilling riser may be several hundred feet or more in length depending on the depth of water at the drilling location. A pressurized fluid, such as water or oil, in the embodiment illustrated, is introduced into the rod ends of the cylinders 26 to provide a tensioning force on the riser. The tensioning force prevents buckling in the riser from motion of the vessel due to the wave action.

As is best shown in FIG. 2 the rod end of the heave compensator cylinder 12 is coupled to the head end of the equivalent riser tensioner cylinder 26e by a liquid line 30, an isolation valve 32, and a pressure sensing safety valve 34. Valve 34 is provided to block liquid from passing out of cylinder 12 should a leak occur in the lines. The isolation valve 32 is positionable to disconnect or interconnect the cylinders 12 and 26e. In the embodiment illustrated gas at a pressure P1 is coupled to an accumulator 43 through a valve 42. The liquid end of the accumulator is coupled to the rod end of the cylinder 26e The gas is contained in a large volume source such as a plurality of bottles 52 maintained at a desired pressure level. A pressure of 2,375 psi on the rod end of the cylinder 26e will provide a riser tensioning pressure of about 700 psi and, through the fluid in the head end of the cylinder 26e, a drill string lifting pressure of 1,675 psi.

Once the desired pressure P] has been set, the heave compensator seeks to retain that pressure and a constant weight is kept on the bit during drilling. The pressure Pl available from the supply will also be continuously adjustable as pipe is added or removed from the drill string and can be easily accomplished by the drilling at a control console. Because of the unique system employed of displacing the hydraulic fluid from the heave compensator to the top end of the riser tensioner cylinders, the drill string load is uneffected by the position of the piston in the heave compensator cylinder and therefore is linear. Loading of the bit at the end of the drill string can thus be made almost as constant as a land based drilling rig with very light variations only at the extreme end of the stroke length of the piston in the compensator cylinder.

It is desirable when removing or adding sections of drill string that the heave compensator be removed from the system. During this operation the drill string is lifted off the bottom of the bore hole so that drilling vessel movements relative to the bore are of no consequence. During the period that the heave compensator is disconnected it is necessary to compensate for the loss of the weight of the drill string from the total combined system. Several alternative approaches could accomplish this result. For example, the heave compensator and the riser tensioner could remain hydraulically coupled and a compensating gas pressure introduced to the head end of the heave compensator cylinder to provide an effective down force equivalent to the lost drill string load. It is more often desirable, however, to re tract the heave compensator piston rod to the top of its stroke so as to remove the piston rod from the working area while the drill string is being added or removed. Thus, in the embodiment illustrated, the piston rod 18 of the heave compensator is allowed to rise to the top of the cylinder 12 after being disconnected from the drill string.

In order to disconnect the drill string load the head end of the riser tensioner cylinder 26e is first coupled to an accumulator 48 through a normally closed valve 46. The gas end of the accumulator is coupled through a valve 50 to a second gas supply 36 at a pressure P2. Pressure P2 is equal to the pressure of the fluid in the heave compensating cylinder or 1,675 psi in the example described and thus compensates for the weight of the drill string DS. Next the valve 32 is actuated to disconnect cylinder 12 from cylinder 26e. The weight of the drill string can then be supported by other means on the derrick and the drill string disconnected. The rotary carrier may then be raised and latched to the traveling block 13.

To reconnect the heave compensator into the system the drill string is again connected to the rotary carrier 20 and the valve 32 is actuated to couple the cylinders 12 and 26e. However, valve 32 cannot be actuated until a fluid balance is restored between the cylinder 12 and the cylinder 26e so that the total fluid in the system is not increased or decreased beyond what it was when the cylinders were disconnected. Otherwise, the pistons could bottom-out prematurely. One solution is to use conventional linear position sensors 62 and 64 on the piston rods 18 and 28, respectively, and by the use of a simple electrical or pneumatic memory system energize valving to reconnect the two cylinders only after the piston rods have been positioned in the same locations as they were when disconnected. Another solution is to manually record the positions of the two piston rods at the time of disconnection and not actuate the valve 32 to reconnect them until the observed positions have been re-established.

Assuming for the purposes of this description that the piston rod positioning is done by operator observation, the valve 32 is then actuated allowing fluid to pass freely between the cylinders 12 and 26e, and finally valve 46 is closed to isolate the accumulator 48.

Other alternate methods of pressure compensation will be apparent to one skilled in the art during the intervals whenthe heave compensator and riser tensioner are both interconnected and isolated from one another. The example illustrated is to be understood as being only one such technique. It should also be understood that the preferred apparatus illustrated is capable of being modified further without departing from the principles of the invention. Thus, the apparatus and pressure compensating systems illustrated are intended to be examples only and that the invention is not to be limited to these specific forms, but rather should be limited only by a literal interpretation of the claims appended hereto.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A combined riser tensioner and drill string heave compensator for a floating drilling vessel having a drilling derrick, a drill string, means for supporting the drill string, means for positioning the drill string supporting means, a riser, and means for supporting the riser, comprising:

drill string heave compensator means coupled to said drill string supporting means for maintaining a substantially constant lifting force on said drill string regardless of movements of the vessel, said drill string heave compensator means including at least one cylinder, piston and piston rod, riser tensioning means coupled to said riser supporting means for maintaining a substantially constant lifting force on said riser regardless of movements of the vessel, said riser tensioning means including at least one cylinder, piston and piston rod,

means for pressurizing hydraulic fluid in said riser tensioning cylinder an amount sufficient to tension the riser and support a predetermined part of the weight of the drill string, means for hydraulically interconnecting said heave compensating cylinder with said riser tensioning cylinder for passing said pressurized hydraulic fluid therebetween to continue said support of the drill string during movement of said drilling vessel,

means for isolating said riser tensioning cylinder from said heave compensating cylinder when said drill string supporting means is disconnected from said heave compensator, and

means for compensating said riser tensioning cylinder for pressure changes due to the loss of the weight of the drill string when said heave compensating cylinder and riser tensioning cylinder are disconnected.

2. The combination of claim 1, said riser tensioning means including a plurality of cylinders, pistons and piston rods equidistantly spaced around said riser.

3. The combination of claim 1, said riser tensioning cylinder compensating means including means for coupling to the head end of the riser tensioning cylinder an independent hydraulic fluid supply of a pressure equal to the pressure in the heave compensating cylinder prior to disconnecting the drill string.

4. The combination of claim 1, said means for pressurizing said rod end of said riser tensioning cylinder including a high pressure air supply, said means for counterbalancing the excess pressure acting on the riser tensioning piston due to loss of weight of the drill string including a lower pressure air supply, and hydraulic accumulator coupled to said gas supplies and to said riser tensioning cylinder.

5. A floating drilling platform in a body of water,

a drilling string,

a riser for the drilling string anchored at an underwater well site and extending out of the water.

fluid-operated riser tensioning means connected to the riser and having a hydraulic expansion chamber varying in size with rise and fall of the platform.

hydraulic heave compensating means for supporting the drilling string having a pressure chamber varying inversely in size with rise and fall of the platform as compared to said expansion chamber,

means selectively hydraulically interconnecting said chambers whereby the vertical position of said drilling string is held stationary relative to said riser.

6. The combination of claim 5, said fluid-operated riser tensioning means including means for disconnecting said hydraulic-interconnection between said chambers, and means for compensating the riser tensioning means for the lost weight of the drilling string during said disconnection.

7. In combination:

a floating drilling platform, first and second vertical telescopic force applying means carried on the platform for respectively supporting a suspended drilling string and tensioning the riser for the drilling string while compensating by their telescopic action for the rise and fall of the platform relative to the drilling string and riser, third means for transferring the force exerted to support the drilling string by the first means as a load for the second means in addition to the riser tensioning load on the latter, fourth means for selectively deactivating said first means, and fifth means for maintaining the same upward effective force exerted by the second means to tension the riser while said first means is deactivated as when it is active.

8. The method of compensating for vertical move ments of a drilling platform floating in a deep body of water while supporting a drilling string and tensioning a riser for the drilling string to prevent buckling, said riser being anchored at an underwater well site and extending to the surface of the water, the steps of constantly tensioning the riser by an upward force exerted by fluid pressure great enough to also support the downward force of the drilling string when applied in direct opposition to said upward force, and hydraulically transferring said downward force into such direct opposition by means of a pressure chamber pressurized by downward pull of the drilling string and hydraulically connected to an expansion chamber whose pressure opposes said upward force, and keeping the collective volumes of said chambers constant during rise and fall of the platform whereby said drilling string is held vertically stationary relative to said riser. 

1. A combined riser tensioner and drill string heave compensator for a floating drilling vessel having a drilling derrick, a drill string, means for supporting the drill string, means for positioning tHe drill string supporting means, a riser, and means for supporting the riser, comprising: drill string heave compensator means coupled to said drill string supporting means for maintaining a substantially constant lifting force on said drill string regardless of movements of the vessel, said drill string heave compensator means including at least one cylinder, piston and piston rod, riser tensioning means coupled to said riser supporting means for maintaining a substantially constant lifting force on said riser regardless of movements of the vessel, said riser tensioning means including at least one cylinder, piston and piston rod, means for pressurizing hydraulic fluid in said riser tensioning cylinder an amount sufficient to tension the riser and support a predetermined part of the weight of the drill string, means for hydraulically interconnecting said heave compensating cylinder with said riser tensioning cylinder for passing said pressurized hydraulic fluid therebetween to continue said support of the drill string during movement of said drilling vessel, means for isolating said riser tensioning cylinder from said heave compensating cylinder when said drill string supporting means is disconnected from said heave compensator, and means for compensating said riser tensioning cylinder for pressure changes due to the loss of the weight of the drill string when said heave compensating cylinder and riser tensioning cylinder are disconnected.
 2. The combination of claim 1, said riser tensioning means including a plurality of cylinders, pistons and piston rods equidistantly spaced around said riser.
 3. The combination of claim 1, said riser tensioning cylinder compensating means including means for coupling to the head end of the riser tensioning cylinder an independent hydraulic fluid supply of a pressure equal to the pressure in the heave compensating cylinder prior to disconnecting the drill string.
 4. The combination of claim 1, said means for pressurizing said rod end of said riser tensioning cylinder including a high pressure air supply, said means for counterbalancing the excess pressure acting on the riser tensioning piston due to loss of weight of the drill string including a lower pressure air supply, and hydraulic accumulators coupled to said gas supplies and to said riser tensioning cylinder.
 5. A floating drilling platform in a body of water, a drilling string, a riser for the drilling string anchored at an underwater well site and extending out of the water, fluid-operated riser tensioning means connected to the riser and having a hydraulic expansion chamber varying in size with rise and fall of the platform, hydraulic heave compensating means for supporting the drilling string having a pressure chamber varying inversely in size with rise and fall of the platform as compared to said expansion chamber, means selectively hydraulically interconnecting said chambers whereby the vertical position of said drilling string is held stationary relative to said riser.
 6. The combination of claim 5, said fluid-operated riser tensioning means including means for disconnecting said hydraulic-interconnection between said chambers, and means for compensating the riser tensioning means for the lost weight of the drilling string during said disconnection.
 7. In combination: a floating drilling platform, first and second vertical telescopic force applying means carried on the platform for respectively supporting a suspended drilling string and tensioning the riser for the drilling string while compensating by their telescopic action for the rise and fall of the platform relative to the drilling string and riser, third means for transferring the force exerted to support the drilling string by the first means as a load for the second means in addition to the riser tensioning load on the latter, fourth means for selectively deactivating said first means, and fifth means for maintaining the same upward effective forCe exerted by the second means to tension the riser while said first means is deactivated as when it is active.
 8. The method of compensating for vertical movements of a drilling platform floating in a deep body of water while supporting a drilling string and tensioning a riser for the drilling string to prevent buckling, said riser being anchored at an underwater well site and extending to the surface of the water, the steps of constantly tensioning the riser by an upward force exerted by fluid pressure great enough to also support the downward force of the drilling string when applied in direct opposition to said upward force, and hydraulically transferring said downward force into such direct opposition by means of a pressure chamber pressurized by downward pull of the drilling string and hydraulically connected to an expansion chamber whose pressure opposes said upward force, and keeping the collective volumes of said chambers constant during rise and fall of the platform whereby said drilling string is held vertically stationary relative to said riser. 